Hi David, hello List,
thank you for your work on FreeDV and Codec2. I'm strongly
interested in a better HAM-DV which must be solve different
demands rather than commercial DV. One major DV-drawback is
the hard decision cutoff at low s/n. While in SSB and even
FM the demodulated s/n is rather proportional of the HF s/n
all DV-systems have a stong threshold. This is a good approach
for commercial but a poor behavior to HAM. We like to use our
brain to listen inside of the channel noise to get the last
sample biological-approximated to what we often already expect
to know or hear (for example in a contest).
Your attempt to use a 2nd carrier points in the right direction.
I've written a suggestion to a HAM-defined DV-system in the german
amateur journal FUNKAMATEUR ("Analoge und digitale Sprachsysteme
für den Amateurfunk"; FA12/12, Page 1260-1263).
http://archiv2.funkamateur.de/search
The idea is to use at least four D-QPSK carriers shifted to minimize
interference. The inner should be modulated with the basic parameters
and a strong FEC. The carriers with more distance to the central
should carry information to enhance the quality and could get a weaker
or at maximum center-distance may be no FEC. The distribution of
parameter bits to the carriers must be carefully choosen out of a list
of significance of your CODEC2 or any other codec. As far as I know
nobody has followed this idea before.
Doing so, the RX then has the chance to dynamically reduce the received
bandwith if the s/n is getting weak. Using only half of the carriers
could enhance the s/n up to 6dB which is very much! Well of cause it
will sound then more "spacy" below the decision point. But this is
much more similar to what we hear on analog SSB or FM if the HF-signal
becoms weak. At least one can listen to the behavior of the cannel and
it's a nice gift to enhance the covering range at the same tramsmitting
power! This is amateur's most wanted feature at all ;-)
This is very different to commercial DV-systems which we currently use
to "recycle" or adapt to an HAM application. Commercial DV needs a
fixed s/n decision due to their cell-design and the quality necessities
of the ordinary telephone user. In fact cell-design is not really
compatible to what amateurs need on short wave. Hence we will never get
below this interesting cutoff level if we don't put dynamic bandwith
adaption to the HAM-DV functional specification. Do you agree?
It would be very nice to do some experiments to find out what one can
win in practice on a real HF-channel.
73, Thomas - DC7GB
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